sd-event: do not assert on invalid signal

The signalfd_siginfo struct is received from outside via a FD, hence
assert() is not appropriate way to check it. Just do a normal runtime
check.

(cherry picked from commit 7a64c5f)

Resolves: RHEL-51171

src/libsystemd/sd-event/sd-event.c

@@ -3866,7 +3866,8 @@ static int process_signal(sd_event *e, struct signal_data *d, uint32_t events, i
                 if (_unlikely_(n != sizeof(si)))
                         return -EIO;
 
-                assert(SIGNAL_VALID(si.ssi_signo));
+                if (_unlikely_(!SIGNAL_VALID(si.ssi_signo)))
+                        return -EINVAL;
 
                 if (e->signal_sources)
                         s = e->signal_sources[si.ssi_signo];